One such system was disclosed by Monnier et al. in U.S. Pat. No. 5,050,962. The system comprises a collimation and combination optical unit joined to an image generator. It also includes a holographic or dichroid mirror, as well as a semitransparent mirror contained in a transparent plate with parallel faces.
The Monnier HMD, however, suffers from several disadvantages: It requires four optical elements as well as a relay lens to make the data image coincide with the focal plane of one of the four elements which has a concave surface with a semireflective mirror coating. Another serious drawback resides in the fact that Monnier, et al. make use of total internal reflection. Optical surfaces that provide total reflection are highly sensitive and the slightest smudge will drastically reduce the reflective efficiency of these surfaces. Also, the rays from the data generator must undergo three reflections before they reach the viewer's eye, which causes a considerable loss of energy. The use of total internal reflection is responsible for a further, quite serious disadvantage of the Monnier device: because of its necessarily much longer optical path inside the device, the field of view of the Monnier HMD is significantly smaller than that of a system of a comparable display size which does not make use of the additional total internal reflection.